Qin Y. Micromanufacturing Engineering and Technology

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framework supporting the tooling elements. This

could be the utilization of a standard die set where

ball-cage bearings ensure the alignment of the

upper and lower part of the die set. The use of a

reference plane, as depicted in Fig. 7-6(a), such as

a ground flat back plate, is another way of ensur-

ing good alignment. By placing the high accuracy

reference element in the middle of the tolerance

chain, it is possible to reference all other tooling

elements to this element. This approach requires

strict control of the tool tolerance chain to ensure

that alignment errors are within an acceptable

level. It is often challenging to ensure that the

macro-size framew ork tooling elements, such as

the stand and the base-plate, are within the preci-

sion required for the alignment of the micro- size

forming tools. In Fig. 7-6(b), the punch is loosely

fitted into a slot of the moving fixture allowing the

punch to center on the die during closure. It is

important that the ali gnment occurs before any

load is applied to the punch, meaning that this

property should be included into the design of

the forming tools. This alignment approach is

simple and self-contained, but is generally only

suitable for simple forming operations of rota-

tionally symmetrical components.

A hybrid solution for the alignment of tooling

elements is depicted in Fig. 7-7. Here the punch

holder fixture is spring loaded with a micrometer

screw, allowing for adjustment of the punch posi-

tion. With this flexible solution it is easy to

achieve good alignment and long-term stability,

and re-establishing the alignment is tolerable.

Flexibility of Tool System. With the vision of

small desktop style factories of the future in mind,

flexibility and low changeover times are impor-

tant factors of focus. With the trend towards

smaller production series and more customized

products, several components should be produced

on the same set-up with a quick change of only the

central forming dies and handling fixtures. This

idea of modularization is easy to grasp and has

been widely adopted in the manufacturing

FIGURE 7-6 (a) Example of a bulk forming set-up where the back-plate is used as a reference for the tooling elements.

(b) A die with a tapered inlet and a flexible punch fixture is used to form a self-aligning bulk forming tool.

FIGURE 7-7 An adjustable punch fixture allows for the

alignment of the punch and die through an adjustment

screw.

CHAPTER 7 Micro-Bulk-Forming 121

industry. When considering micro-bulk forming,

it is suggested to realize a flexible bulk forming

system through the utilization of modified normal

machine elements.

Figure 7-8(a) depicts an example of a two-step

flexible micro- forming set-up. The forming dies,

punches and ejectors are manufactured from stan-

dard punch needles. By standardizing the diame-

ter and tooling material and allowing for easy set-

up, the central tooling elements can easily be

changed and the machine can manufacture differ-

ent components with low change times. With an

outer diameter of 8 mm and a ground surface

finish, the forming dies in Fig. 7-8(b) can be easily

interchanged.

Warm Forging of Micro-components

Micro-components are often manufactured using

precious metals such as alloys of palladium, tita-

nium or magnesium. The choice of mat erial is

often given by the end applications, where the

environment of use can be humid, corrosive or

have special demands for strength or biocompat-

ibility. Also new age materials, such as bulk

metallic glasses, are expected to find wide use

within micro-bulk forming. With some of these

advanced materials it can be beneficial to utilize

warm forging, where the central forming tool ele-

ments are operating at elevated temperature. The

main benefits of warm forging are a decreased

tooling load, increased ductility of the workpiece

material and elimination of the heat treatment

process prior and post to forging. For some mate-

rials, such as bulk metallic glasses or high grade

titanium, it is required to warm the forming tools

in order to have an acceptable lifetime of the tool-

ing elements. Warm forging is defined as forging

at elevated temperatures above the ambient tem-

perature and below the material recrysta llization

temperature, typically in the range 100–400



The use of warm forming tools requires an

advanced tooling set-up with good thermal isola-

tion, heat shields and possible external cooling. It

is essential to control the temperature gradient

between the warm forging tool elements and the

cold framework element s to minimize thermal

defections and the following misalignment and

possible tool damage. Figure 7-9(a) depicts a pro-

totype tool system for warm forging of a dental

implant in titanium.

The system consists of a central heated core

including die, punch, ejector supporting elements

and heaters. The framewo rk die set is thermally

isolated from the warm core by a calcium silicate-

based ceram ic material with a thermal conductiv-

ity of 0.4 W/mK. Heat shields, with an optical

reflective finish, are placed around the upper

and lower heated tooling elements. These heat

shields prevent heat transfer to the frame by radi-

ation and have a secondary function as a simple

safety measure for avoidance of tou ching of the

warm tool parts. Figure 7-9(b) illustrates the

steady state temperature distribution of the tool

system when heated to 200



C. It can be observed

that the tool frame is kept cold with only an

FIGURE 7-8 Sectional view of a two stage micro-bulk forming machine.

122 CHAPTER 7 Micro-Bulk-Forming

insignificant temperature gradient over the sup-

porting tool frame, thereby keeping thermal

defections to an acceptable level. In an industrial

set-up it is usually necessary to apply external

temperature control as known from tools for ther-

moplastic injection molding.

The elevated temperature in warm forging

imposes some additional design challenges on

the tooling parts located in the heated zone of

the tool. For an open-die forging scenario the pri-

mary adjacent factors to handle are more chal-

lenging tribological conditions. The tribological

conditions can be controlled by applying suitable

coatings to the punch and die, and matching the

workpiece material and the lubricant: the section

on tribology later in the chapter touches more on

these issues. Further, periodically cleaning of the

punch and die might be needed to remove residues

of lubricant and workpiece material pickup. For

closed-die forming the tribological challenges

remain and are followed by oxidation and lubri-

cant entrapment issues. When tempering the

punch and die at temperatures near to the recrys-

tallization point, oxides will form on the surface

of these parts. The oxide layer, having a high

friction coefficient, will give rise to a narrowing

of tool clearances and increased friction between

the tool pieces and between the tool and work-

piece. This can lead to tool breakdown and should

be avoided. Further, pickup due to lubricant

breakdown and lubricant entrapment is more

dominant at elevated temperatures. Most lubri-

cants are unfit for warm forging because the base

liquid components become volatile, leaving solid

lubricant residues in the forming die and on the

workpiece surface. However, by application of

the right coatings in combination with the right

type and amount of lubricant, it is possible to

realize a micro- bulk forming process at elevated

temperature.

Handling and Ejection System

Today, most industrial machin es handling micro-

components are based on conventional circular

vibrating screeners. These screeners are cross-

vibrating vessels that are able to align a specific

component geometry by means of a custom-built

mechanical gate system allowing only compo-

nents that are rightly aligned to pass through.

Depending on the component geometry, material

and the crafted design of the gate system, the

mean failure rate of a vibrating screen system is

usually around l = 1E-2 to l = 1E-6 pieces, mean-

ing that one in every hundred to every million

components will jam the system so that an

FIGURE 7-9 (a) A prototype tool system for warm micro-bulk forming of a dental implant in titanium. (b) Steady state

thermal analysis of tool system including external die set for alignment.

CHAPTER 7 Micro-Bulk-Forming 123

operator will have to inspect the system. Further,

the vibrating screeners are bulky, noisy and inflex-

ible, making them unsuitable as a handling system

for a multi -stage micro-manufacturing machine.

The sticking of matter is another issue that influ-

ences the handling of micro-components. With

the increased surface-to-volume ratio for these

small compon ents, the van der Waals forces will

cause sticking of the components. Primarily, the

components will tend to stick to each other,

requiring a strategy capable of both handling

and separation. For this reason handling con cepts

of micro-components have been studied inten-

sively in recent years [12,13]. The use of robot

cells, advanced gripper systems and self-assembly

systems have been proposed, but none have been

widely employed within micro-manufacturing

because of the unfavorable combination of cost,

complexity and speed of these systems.

Figure 7-10 depicts a handling system for the

micro-bulk forming process. A cut-out of the

transfer system is illustrated in Fig. 7-10(b), where

the components can be ejected into the container

fixture, moved laterally to the next station and

inserted into the succeeding forming die. This sys-

tem is capable of transferring components at high

speed and is simple to monitor for failure. The

container is a precision element used to hold and

transport the components from one forming

station to the nex t. For the container depicted in

Fig. 7-10(a), the component is held in place by a

combination of friction forces and surface tension

forces. In order for the surface tension forces to

overcome the gravitational forces and keep the

component in the container, a specific clearance

in the order of micrometers between the compo-

nent and the container wall must be realized.

Furthermore, wear of the container, production

tolerances and lubrication quantity must be under

control.

Another concept for the component to be held

in place in the transfer container is shown in

Fig. 7-10(b). In this design an O- ring is pl aced

inside the container die and primarily functions

as a friction gate. Secondarily the O-ring also has

a centering function because the rubber O-ring

will absorb minor misalignment and center the

component in the container. The proposed trans-

fer system is only capable of handling rotation-

ally symmetrical components, but could be made

to function for fre e-form w orkpiece geometries

also.

FIGURE 7-10 (a) Gripper for cylindrical micro-component functioning by a combination of friction and sticking forces.

(b) Gripper with integrated rubber O-ring. The billet is centered in the carrier and is held in place by the friction of the O-ring.

124 CHAPTER 7 Micro-Bulk-Forming

Process Supervision

Intelligent process supervision is an area of

increasing interest within manufacturing tech-

nology. By online monitoring of different process

parameters and real-time comparison with

a priori values or values acquired through earlier

processing, it is possible to make sure that the

process is performing correctly. Some com-

mon parameters that are monitored in a bulk

forming process are the load/stroke curve for each

forming cell, the tool temperature, the workpiece

weight, and the position of the trans fer rig.

The micro-bulk forming tool shown in Fig. 7-11

employs online measurement of load on the indi-

vidual forming cells by piezo force sensors. The

tool set-up is equipped with two piezo-type force

transducers which measure the forming and

ejection forces on each of the two forming cells.

By correlating the forming force with the stroke

length, given by the actuator position encoder,

the load/stroke curve can be determined and

the curve can be compared with some pre-

established curve envelopes. With this type of

set-up it is possible to detect machine errors

such as tool breakdown, stuck workpieces and

misplaced aligned workpieces due to transfer

errors. Secondary faults such as inconsistency

in workpiece m aterial, lubricant breakdown

and tool wear can also be monitored.

Data logging of the monitored values can serve

as a quality assurance parameter of the manufac-

tured components. As it is not uncommon to

encounter inspection requirement rates of 100%

within micro-manufacturing, the addition of

traceability and online qua lity inspection can be

valuable. Furthermore, the ability to stop the bulk

forming machine in case of misalignment of work-

piece or general tool malfunction can avoid

machine breakdown, save tooling costs and help

to minimize production downtime.

MICRO-TRIBOLOGY

Tribology is the science and technology of inter-

acting surfaces in relative motion. This includes

the study of the phenomena of friction, lubrication

and wear phenomena. Micro-tribology is the sub-

category of tribology dealing with the interaction

of micro-size surfaces. As mentioned in the earlier

section on the size effect, the laws of friction and

lubrication are prone to the size effect. The main

reason for this can be found in the underlying

surface roughness (or surface asperities, as it is

usually termed when referring to tribology), which

only scales to a certain extent. This means that the

relative surface roughness of a micro-component

will be greater in the case of a micro-component

compared with macro-size. However, the main

parameter of interest in bulk forming is friction,

which is dependent on surface load, lubrication

and surface characteristics, including surface

roughness. Amortons’ law of friction dictates a

linear dependence between load and friction force:

¼ mL,whereF

donates the friction force, m is

the friction coefficient and L is the load. It can be

noted that the friction force is independent of con-

tact area, something that was later challenged by

the Bowden and Tabor law of friction. Here the

friction force F

is expressed at the product of the

effective shear stress, denoted by t,andthesum

of the areas of the asperities in contact:

¼ t

asp

. In bulk forming, the friction coef-

ficient m is often established on the basis of exper-

iment and there is seldom an explicit formulation

for this quantity. The double-cup extrusion test

(DCE test) is a recognized way of establishing

FIGURE 7-11 Online measurement of load on the indi-

vidual forming cells by piezo force sensors.

CHAPTER 7 Micro-Bulk-Forming 125

the friction coefficient experimentally. The exper-

imental set-up, depicted in Fig. 7-12, is based on a

double-cup pressed between two punches with

equal geometry. In the case of zero friction, the

height ratio between the upper and lower cup will

be unity, while no lower cup will be formed in the

case of infinite friction.

A number of micro-scaled DCE tests were

carried out by Engel and it was found that the

measured friction coefficient depends on the scale

of the experiment [14]. It was further found that

the friction coeffic ient would increase by a factor

of 20 when the experiment was scaled by a factor

of 8. According to Engel, this is due to the fact that

for micro-scale surfaces, more surface asperities

reside close to the boundaries of the workpiece

where they are less likely to form lubricant pock-

ets under hydrostatic pressure. This influenc es the

surface contact area, leading to an increase in

friction force, as observed from the Bowden and

Tabor law of friction. Mo re studies of friction

behavior can be found in references [15,16].

The influence of increase in friction coefficient

when working in the micro-size domain brings

about challenge s for the handling and ejection of

the bulk formed micro-components. Considering

the increase of the surface-to-volume ratio, by the

scaling factor a, while keeping in mind that sur-

face area promotes friction and volume of mate-

rial brings strength, this is an important frictional

challenge encountered within micro-bulk form-

ing. Further, with the increase in friction due to

open lubricant pockets for the micro-size domain,

it is evident that friction is a key challenge to be

overcome in micro-bulk forming.

Figure 7-13(a) illustrates a simulation of a pre-

form for a component to be manufactured by

micro-bulk forming. In this case the lower pin

FIGURE 7-13 (a) 3D simulation of pre-form for a micro-axle. (b) Photograph of the formed component where unwanted

reverse forming has occurred during ejection from the forming die.

FIGURE 7-12 Sectional illustration of the double-cup

extrusion test.

126 CHAPTER 7 Micro-Bulk-Forming

extrusion, with a diameter of about 0.5 mm, must

be able to withstand the total friction force during

ejection. If the friction force is greater than the

yield strength of the component the pin will col-

lapse and reverse forming will occur. The photo-

graph in Fig. 7-13(b) presents an example of

reverse forming of the component: this compo-

nent is the realization of the simulation illustrated

in Fig. 7-13(a) where the forward rod extrusion

has collapsed during ejection. The problem was

resolved by a reduction of the press load, change

of lubricant and polishing of the forming die.

The use and application of lubricants for micro-

bulk forming is at the present time not very

well researched. In convent ional bulk forming,

soaped-phosphate lubricant has been the lubri-

cant of choice for several decades. Unfortunately

this lubricant is unsuitable for micro-bulk form-

ing due to the chemical properties of the lubricant

layer, allowing scaling down only to a certain

thickness. In the example discussed above and

other mi cro-bulk forming experiments at ambient

temperature, the use of a commercial silicon paste

showed good results. However, the application

and removal of the lubricant as well as unintended

confinement of lubricant in the forming die have

been identified as challenges.

For warm forging and the forming of bulk

metallic glasses, a commercial sprayable lubricant

based on MoSO

has been utilized. This lubrica-

tion approach worked fairly well and is suitable for

forming at elevated temperatures. A drawback of

utilizing a MoSO

-based lubricant is the undesir-

able interaction with the workpiece surface. This

will leave a dark-colored, rough surface which is

difficult to remove and is generally unsuitable for

use in any advanced or medical applications.

PROCESS ANALYSIS

The analysis of the bulk forming process normally

takes place in a dedicated finite element method

(FEM) computer simulation environment. In

practice, the software is solving the underlying

partial differential equations by doing incremen-

tal integration of an approximate set of equations.

Due to the severe deformation of the workpiece in

bulk forming, the software must be capable of

doing accurate remeshing while retaining vo lume

constancy. The process analysis is initiated by

establishing the material parameters. The work-

piece material flow curve is typically acquired by

doing a tensile test or performing an upsetting

test in a universal testing machine. Once the

material data are known, these can be fed into

the material model of the FEM software together

with the tool geometry. If the material is strain-

rate dependent or the process needs to run at high

strain rates, this influence will have to be incl uded

in the model also. This forms the physical basis of

the simulation and the simulation process pro-

ceeds by selection of simulation parameters.

The choice and analysis of simulation para-

meters depend on the type of forging process,

tolerances, simulation accuracy and several other

influencing fact ors. The most important factor,

however, is the number of elements in the simu-

lation, N. The N-factor determines the number of

nodes in the mesh and is thus responsible for the

simulation accuracy. The numb er of calculations

to be done by the computer increases linearly with

N, and thus the number of elements becomes a

trade-off between calculation workload and sim-

ulation accuracy. It is possible to use a zoned mesh

where the user can de fine zones of the workpiece

to have a more refined mesh, typically in areas of

severe deformation.

Another important simulation parameter is the

simulation step size. This is the time (or punch

travel distance) between successive simulation

steps. As in the case of number of elements, a large

number of steps indicate a large calculation work-

load, while a low number of simulations steps will

affect simulation accuracy. However, by taking

notice of the rate of convergence of the simulation

it is possible to choose the right step size within a

few iterations.

For rotationally symmetrical workpieces, such

as bolt, axle or cup geometries, it is possible to

cut down on the calculations by doing a two-

dimensional simulation while retaining the full

validity of the simulation. Even in the case of

three-dimensional geometries it is often possible

to impose symmetry constraints and cut down on

CHAPTER 7 Micro-Bulk-Forming 127

the amount of calculations or increase the accu-

racy of the simulation. Figure 7-14(a) illustrates

the achieved geometry and stress distribution of a

two-dimensional simulation of a double-cup

extrusion. The simulation was done with a billet

diameter of 1 mm of CuZ n15 material and a fric-

tion coefficient m = 0.15 between the billet and

container wall. Figure 7-14(b) is a plot of the

corresponding load/stroke curve where the expo-

nential progress of the envelope can be attributed

to the work hardening of the workpiece material.

The benefits of simulations are manifold and

simulations of traditional bulk forming processes

often have exhibited a geometrical accuracy of

better than 0.5%. If the simulation parameters

are chosen carefully it is possible to predict the

press load, unwanted forming folds and tool stress

without having to prepare costly prototype tools.

The micro-bulk forming proces s can be simu-

lated in the same way as conventional macro-size

bulk forming process with a few important excep-

tions. Most simulation models hold an underlying

assumption of a continuous and isotropic mate-

rial model. This means that the grain structure of

the material is not taken into account, meaning

that the model is only valid down to a certain size.

Within normal limits, a material can be modeled

as continuous and isotropic if it is indeed isotropic

and the number of grain structures across all fea-

tures is greater than 8–10 grains [15].

Figure 7-15 illustrates a forming experi-

ment where a small copper pin was extruded.

FIGURE 7-14 FE simulation of a double-cup extrusion: (a) stress distribution, and (b) load/stroke curve.

FIGURE 7-15 A sample of the extruded micro-pin (dimensions: mm).

128 CHAPTER 7 Micro-Bulk-Forming

The original specification is reproduced in

Fig. 7-15(a). With an average grain size of about

80 mm, there are less than four grains across the

pin, thereby invalidating the assumption of isot-

ropy of the material. A photograph of the bulk

formed pin is shown in Fig. 7-15(b) where a clear

side tracing of the extruded pin can be noted. This

behavior has been the topic of several research

projects, and a range of new material models try-

ing to incorporate the inter-grain behavior have

been proposed. By choosing a material with a fine

grain structure or by changing the grain size by

heat treatment or mechanical remodeling, it is

possible to form micro-size components while

keeping the assumption of homogeneity and isot-

ropy in the mat erial intact.

REFERENCES

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induced during cold forming of stainless steel sec-

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(2008) 1310–1316.

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enhancement of the corner regions of stainless steel

cross-sections, J. of Constructional Steel Research 61

(1) (2005) 37–52.

[3] T. Inoue, The Japanese sword: the material,

manufacturing and computer simulation of quench-

ing process, Materials Science Research International

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N. Bay, Development of a flexible tool system for

small quantity production in cold forging, J. Mater.

Process. Technol. 71 (11/1) (1997) 36–42.

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research to its realization, J. Mater. Process. Technol.

125–126 (2002) 35–44.

[6] M. Geiger, M. Kleiner, R. Eckstein, N. Tiesler,

U. Engel, Microforming, CIRP Ann. Manuf. Technol.

50 (2001) 445–462.

[7] J.A.Wert,C.Thornsen,R.D.Jensen,M.Arentoft,

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CHAPTER 7 Micro-Bulk-Forming 129

Forming of Micro-Sheet-

Metal Components

Yi Qin, Andrew Brockett, Jie Zhao, Akhtar Razali,

Yanling Ma and Colin Harrison

INTRODUCTION

Sheet metal components are used extensively

in various applications such as vehicles, aircraft,

electronics products, medical implants and

packaging for consuming goods, typical parts/

components i ncluding car panels, aircraft

skins, cans for food and drinks, frames for TV/

computer screens/monitors/displays, etc. Con-

cerning miniature/micro-products, sheet metal

parts include electrical connectors and lead

frames, micro-meshes for masks and optical

devices, micro-springs for micro-switches, micro-

cups for electron guns and micro-packaging,

micro-laminates for micro-motor and fluidic

devices, micro-gears for micro-mechanical de-

vices, casings/housings for micro-device assem-

bly/packaging, micro-knives for surgery, etc.

Therefore, miniature/micro-sheet metal parts are

closely associated with everyday life.

Basic process configurations for the forming of

macro-products include shearing, blanking, bend-

ing, stamping, deep drawing (including mechanical

and hydromechanical), hydroforming, stretching

forming, super-plastic forming, age forming, spin-

ning, explosive forming, incremental forming, etc.

Some of these processes may be equally applied to

the forming of miniature and even micro-products,

if the issues related to ‘size effect’ can be handled

successfully [1–8]. General challenges associated

with the manufacture of micro-products have been

described in Chapter 1 of this book.

The forming of small/thin metal parts has

been undertaken by industry for many years.

New challenges arise when the overall sizes

reduce to sub-millimeters, or local features reduce

to tens of microns, or the precision requirements

for macro-/miniature parts reduce to less than a

few microns. Studying the research reported pre-

viously and recent research condu cted in-house

resulted in the following observations:

1. Conventional metal-forming process config-

urations may be equally used for the form-

ing of miniature/micro-parts, although the

process capabilities are likely to be con-

strained more, due to additional material,

interfacial and tooling c onsiderations in

micro-forming.

2. The types of materials which could be formable

at micro-levels are prescribed more signifi-

cantly than for forming at macro-levels by the

micro-structures and grain-boundary proper-

ties of the materials. The forming limits for

these materials are, therefore, somewhat differ-

ent, compared to those for the forming of

macro-parts.

3. Size effects may exist in material/property and

tool/material interfacial property characteriza-

tion, depending largely on the micro-structures

of the materials, which leads to the require-

ment of the definition of these parameters with

reference to the actual materials an d interfaces

to be used.

CHAPTER

130